In spite of these extensive studies, resistance of lobsters to the Aerococcus has been obtained only rarely. In one experiment reported by Stewart, filter sterilized serum from heavily infected lobsters was injected into healthy animals in massive amounts; this had no adverse effect but apparently was not studied to determine if any immunity was conferred. The lack of infectivity is, by the same author, indicated as possibly attributable to a lack of virulence or a change in virulence of the pathogen. A limited degree of resistance has been observed upon injection of vaccines prepared from formalin-killed cells of a virulent strains grown both in vitro and in vivo, but only a low level of protection was achieved.
The best protection heretofore obtained has been achieved by injecting the lobsters with an initial dose of vancomycin, followed by injection of live pathogen cells 24 hours later. However, this method has a serious drawback in that the amounts of vancomycin used are over a thousandfold greater than in the vaccine of the present invention and the resultant lobster meat is accordingly contaminated with residual amounts of the antibiotic.